This application claims the priority of German Patent Application Serial No. 101 19 839.6, filed Apr. 23, 2001, pursuant to 35 U.S.C. 119(a)-(d), the subject matter of which is incorporated herein by reference.
The present invention relates, in general, to a method of making an axle element for a motor vehicle, and to a shaping die for carrying out the method.
Axle elements for motor vehicles, in particular swivel bearings for the front axle, constitute fairly complex single-piece components which are cast or forged and are made of a cast steel material or aluminum. As swivel bearings are exposed to severe forces, they have generally been manufactured of solid material. Weight concerns made it increasingly likely to manufacture swivel bearings of light metals such as aluminum.
Swivel bearings of aluminum are typically made through a forging process by using a blank in the form of an extruded round stock. A problem associated with this approach is the fact that finished swivel bearings have a very irregular weight distribution which, however, is not reflected in the cylindrical shape of the blank. In order to still provide those regions that require more mass than other regions with enough material for the forging process, it has been proposed to use sufficiently sized blanks. This approach suffers also shortcomings in view of the substantial energy consumption required to heat the blank to the desired forge temperature for the hot forming process. Moreover, the volume of encountered waste is relatively high in those regions of lesser mass accumulations. This adversely affects costs considerations, especially when taking into account that aluminum is a comparably expensive material. These problems are not only true for swivel bearings but apply for axle elements in general.
It would therefore be desirable and advantageous to provide an improved method of making an axle element for motor vehicles, to obviate prior art shortcomings and to reduce the energy amount required for the forming process and the material consumption required for the manufacture.
According to one aspect of the present invention, a method of making an axle element for a motor vehicle, includes the steps of heating a rod-shaped semi-finished product of aluminum to a desired shaping temperature; upsetting one end of the semi-finished product in a cavity of a shaping die by means of a first punch; compressing another end of the semi-finished product in the cavity by a second punch and thereby forcing compressed material into a branch of the cavity to form a pre-forge part with a leg portion; and forging the pre-forge part into a finished axle element.
The present invention resolves prior art problems by altering the even mass distribution of the rod-shaped semi-finished product, in particular round stock, in such a manner with respect to the axle element to be manufactured, that in a pre-stage of the actual forging process more material is made available in regions of greater material accumulations than in other regions. As a consequence, a mass distribution is realized which resembles the mass distribution of the axle element to be manufactured.
Upsetting is realized by providing the shaping die with a cavity which has an internal shape to permit a material thickening in discrete regions. Depending on the internal shape of the cavity, one end of the semi-finished product may be compressed directly by the punch. The cavity may also have an impression at a distance to the one end of the semi-finished product for receiving displaced material as a result of compressive deformation during the upsetting process. The geometric configuration of the cavity is determined under consideration of certain diametrical ratios and/or length ratios of compressed regions. To prevent surface creases, the length of compressive deformation should not be greater than five times the diameter of the semi-finished product. Upsetting is implemented by advancing the semi-finished product to the first punch and moving the punch at a speed of up to 150 mm/s along the predetermined length of compressive deformation.
Once the one end of the semi-finished product has been upset, the second punch acts on the other end of the semi-finished product to press it into the cavity. Compressed material flows backwards into the branch of the cavity, whereby the branch extends at an angle to the longitudinal center axis of the rod-shaped semi-finished product. Hereby, the compressed material is shaped into a leg portion of the semi-finished product. The branch may be situated halfway of the length of compressive deformation. Suitably, the cavity is so configured that a neutral axis is defined in the area of the longitudinal center axis of the formed leg portion. This ensures that the neutral axis flows into the flash in the following final forging step in the forging die to thereby provide an unobjectionable and aligned texture of the structural element. The recrystallization behavior of the neutral axis is influenced by the parameters, punch speed, temperature of the shaping die and the semi-finished product, geometry of the cavity, and friction on the cavity surface. Recrystallization can be reduced to a minimum through optimized selection of these parameters.
In accordance with the present invention, a single shaping die can be used to combine two different processes. On the one hand, the end of the semi-finished product can be upset, and, on the other hand, a leg portion can be formed through compressive deformation, without requiring a transfer of the rod-shaped semi-finished product between these two manufacturing steps. Once the semi-finished product is transformed into a pre-forge part, the final forging process is carried out to produce the finished product, e.g. axle element.
The method according to the present invention has many advantages. Semi-finished products, such as extruded round stock with slight initial cross section, can be used and shaped to have a beneficial mass distribution for the forging process. At the same time, the energy consumption for heating the semi-finished product is reduced as a consequence of the small mass of the semi-finished products. Moreover, flash losses during the forging process are smaller because the pre-forge part has already a configuration which close resembles the final configuration, so that less initial material is required.
The upsetting forces and compressive forces for backflow of material are suitably applied in horizontal direction to simplify the overall construction of the shaping die. In order to keep the shaping die closed during the deformation process, the clamping force acting on the shaping die should be about ten times the force applied by the first and second punches.
According to another feature of the present invention, the other end of the semi-finished product is pressed in the cavity by the second punch against a restraining force applied by the first punch. Thus, the first punch assumes a dual function, namely upsetting one end of the semi-finished product and providing an abutment for operation of the second punch, without loss of time or need for repositioning the workpiece for initiating the compressive operation by the second punch. In a same way, the second punch provides an abutment during upsetting operation of the first punch.
While blanks are heated during hot forging to a temperature above the recrystallization temperature to avoid a remaining hardening of the workpiece material, it is sufficient to heat a pre-forge part of aluminum during manufacture to a shaping temperature of less than 520xc2x0 C., even as low as between 420xc2x0 C. and 480xc2x0 C. Shaping at a too low temperature may cause a dislocation density not yet degraded, that may lead during following heating to forge temperature to an uncontrolled and undesired coarse grain formation through recrystallization processes.
According to another feature of the present invention, the upsetting step is repeated in a cavity of a further shaping die before the forging step. This may be advantageous in those situations in which the upsetting forces applied by the first punch should not exceed a predetermined level so that material is prevented from an uncontrolled flow into the branch of the cavity but enters the branch in a controlled manner only during the compression step by means of the second punch. The provision of a second shaping die enables an even closer configuration of the pre-forge part to the final configuration.
According to another feature of the present invention, the pre-forge part is heated to a forge temperature before the forging step. In this way, a permanent hardening of the workpiece material is prevented. Suitably, the forge temperature is above the recrystallization temperature, e.g. about 520xc2x0 C.
The method according to the present invention is applicable for the manufacture of differently configured axle elements, in particular the production of swivel bearings.